Urban Forests and Pollution

Written by Melanie Lenart, University of Arizona
Urban trees play multiple roles when it comes to local air pollution. While trees in general help reduce air pollution, including absorbing the greenhouse gas carbon dioxide, some species contribute to local smog by emitting volatile organic carbons (VOCs). Planting locations of individual trees and species selection make a difference in the overall pollution balance.

Many urban trees help clear the air of pollution, although some are better at it than others (Table 1). Trees remove some of the gases in the air, including ozone, carbon monoxide, and nitrogen dioxide, by absorbing them through their stomata and on their leaf surfaces (Nowak 2000). All trees collect carbon dioxide, which was designated as a pollutant because of its contribution to climate change. Trees also intercept sulfur dioxide and the fine airborne particles so troubling to asthmatics.

Large trees remove far more pollutants than small trees. A 30-inch-diameter tree removes about 70 times more air pollution a year than a 3-inch-diameter tree, assuming both are healthy (Nowak 1994). Trees flourishing in New York City's Central Park and along curbs removed about 1,800 metric tons of air pollution in 1994 (Nowak 2000). It remains difficult to account for the value of urban trees in preventing dust and other pollution in upper-level winds from reaching the surface below the canopy.

However, some trees contribute to the production of ground-level ozone, a main component of smog. Certain tree species emit VOCs that can contribute to the formation of ground-level ozone in the presence of nitrogen oxides (NOx) from fossil fuel emissions, such as auto exhaust. (See Benjamin et al. 1996 for average emission rates for hundreds of species.) Management options, such as species selection for future planting, can help limit this problem, as not all species emit VOCs (Table 1).

Trees evolved the ability to emit VOCs during earlier warm periods on the planet when carbon dioxide levels were high. Evidence indicates the VOCs increase the heat tolerance of these trees (Russell Monson, pers. comm.). With the arrival of cars that emit NOx gases, this apparent adaptive mechanism became a problem because these two can interact to produce ozone which can do extensive damage to urban trees, and even distant forests far from the city source (Appleton et al. 2000).

Other confounding factors, including temperature, make it challenging to determine the overall effect on pollution of VOC-emitting trees. For one thing, urban trees or any plant species can reduce smog levels by cooling the nearby environment. One study estimated that shade trees could reduce the maximum smog concentration by 5% – equivalent to a 12% reduction in smog precursors, such as VOCs – by evaporatively cooling and shading the city environment (Akbari et al. 2001). Trees tend to emit more VOCs as temperatures rise, and smog alerts generally occur on hot days. Even in smog-prone Los Angeles, the authors note, ozone concentration did not reach health-damaging levels as long as air temperatures remained below 70°F.

Another confounding factor in predicting future VOC emissions from trees relates to the ongoing rise in carbon dioxide levels from car exhausts and other sources of fossil fuel burning. The increase in carbon dioxide that is increasing temperatures also tempers VOC production in species for which this was compared, such as aspen (Populus tremuloides) and sweetgum (Liquidambar spp.). In a detailed analysis modeling an increase in carbon dioxide by 200 parts per million coupled with a rise in temperatures of 3°C, Heald and colleagues (2009) estimated that emissions of the VOC isoprene would remain roughly the same around the world.

Table 1. Some tree species with a recognized capacity for removing airborne pollution (including ozone, carbon monoxide, particulate matter, sulfur dioxide, and nitrogen dioxide) or contributing to it by emitting isoprene, a prominent volatile organic carbon (VOC) that can contribute to ground-level ozone formation in the presence of anthropogenically produced nitrogen oxides.

*Species (or genus with species) with a tolerance to ozone, as identified by Appleton et al. (2000). +Species (or genus with species) that show a sensitivity to ozone, as identified by Appleton et al. (2000).